This project will probe the role of purine nucleoside and nucleobase transporters (NTs) in the biology of Leishmania parasites. All parasitic protozoa examined to date are unable to synthesize purines and must salvage these essential nutrients from their hosts. The first step in salvage is the uptake of purine nucleosides or nucleobases across the parasite plasma membrane by NTs. There are three Specific Aims. The first Specific Aim is to study the structure and function of a model nucleoside transporter from L. donovani, LdNT1.1, to elucidate how nucleoside transporters function in Leishmania and in other parasitic protozoa. The first component of this Specific Aim is to analyze 18 previously identified missense mutants of LdNT1.1 that are non-functional in transport or have altered substrate specificity. These mutants will be examined for those that are non-functional because they fail to bind substrate and those that are still able to bind substrate but unable to translocate it across the plasma membrane. These experiments will identify amino acids that play crucial roles in substrate recognition and binding or in substrate translocation and will thus define components of the permease that promote its two essential mechanistic functions. The second component of Specific Aim 1 is to define extracellular and intracellular gating residues for LdNT1.1, functionally important residues that interact to close off the permeation pore in the inward-facing and outward-facing orientation respectively. The second Specific Aim entails the creation of a structural model of LmaNT4, a unique nucleobase transporter from L. major that functions optimally at pH 5, the intracellular pH of the macrophage phagolysosome where the parasite resides in the mammalian host, and the genetic dissection of key residues that likely control this unusual pH optimum. The third Specific Aim is to examine the molecular mechanisms L. donovani employs to adapt to purine starvation. Preliminary evidence reveals that the LdNT2 protein is dramatically upregulated upon purine starvation whereas the level of its mRNA is unaffected. Planned experiments will determine whether this regulation operates at the level of translation, protein turnover, or both. The potential regulation of the other LdNTs by purine limitation will also be examined.